Data compression is a technique that enables data to be coded in order to minimize the number of bits required to represent the original data.
Adaptive lossless data compression (ALDC) is a technique, as the name suggests, that enables the compression to be performed in a dynamic manner without any data being lost, thus enabling the original data to be regenerated to exactly its original state during a decompression operation.
An ALDC system typically uses a content addressable memory (CAM), which comprises a history buffer that stores a dictionary of data sequences. Incoming file strings to be compressed are adaptively matched against the data stored in the history buffer, such that the original data is represented by a succession of matches with the dictionary.
Existing CAM based ALDC compression engines require match signals (or flags) to be created at each history buffer location. The match signals are often combined logically, for example OR'd together, with the result being used to drive out a control signal to all locations of the history buffer.
This means that in one clock cycle there can be a large fan-in of signals to an OR gate, and a large fan-out of signals back to all history buffer locations in a cascade arrangement. For compression engines with relatively small history buffers this does not present any issues. However, for large compression engines the large fan-in and fan-out can be disadvantageous.
For example, if the size of a history buffer is increased, for example from 1024 to 16384 bytes (i.e. a 16 times increase), current technology does not allow for fan-in and fan-out to be achieved in one cycle for such a history buffer.
Another disadvantage with existing Cam based ALDC engines is that a delay can exist between the completion of the detection of one match sequence and the commencement of the detection of a new match sequence. This has the disadvantage of reducing the compression ratio.